According to earlier methods, electrodes were applied to the head to reveal the extent of stretching and contraction of the muscles of the eyes when they move so that the amplitude of eyeball movement can be determined, but these methods are delicate to perform and they have only a limited scope.
In order to overcome these shortcomings, differential photoelectric nystagmagraphic instruments using visible light were built separately by Prof. N. TOROKS, University of Illinois, Chicago, USA (1951, 1955) and by C. R. PFALTZ and H. R. RICHTER who recorded nystagmus to 0.1 of a degree (International Review of Otolaryngology, Vol. 18, No. 4, 1956, New York). The method works on the principle that the sclera (i.e. the white of the eye) is a much better light reflector than the iris (i.e. the transparent portion of the eye). Thus, photocells were placed so that their medial edges aiming at the medial and lateral junctions of the cornea and iris respectively with a source of central light interposed centered on the pupil, would lead to an intensity variation of the light reflected by the eye's surface.
The intensity variations can be picked up differentially by 2 or 4 photocells mounted, for example, on goggles and are led to a current EEG recorder. Using 4 photocells, the eye movements both in a horizontal plane and a vertical plane can be recorded simultaneously by 2 channels of the recorder. The magnitude of the recorded nystagmic jerk was found to be proportional to the eye movement and its linear increase was proved. The method is technically simple, its sensitivity is high and it can be used for recording spontaneous caloric, rotational, galvanic as well as positional nystagmus.
In the course of 8 years, these authors improved the method; they replaced the visible central illumination by invisible infrared light centered on the pupil to eliminate the effect of light adaptation and irritation and consequently rapid tiring of the subject. (Annals of Otology, Rhinology and Laryngology, Vol. 73, No. 4, page 893, December 1964). Thus a nystagmus jerk as small as 0.5.degree. could be accurately measured; there is a linear increase in the amplitude of the recorded spikes up to an eye movement of 20.degree., which exceeded the limits found in clinical nystagmus.
Such a principle was also applied by NASA in the space program (NASA Tech Brief 65-10079, March 1965): a device whereby an astronaut, precluded from using his hands, is capable of starting an external relay from a distance simply by voluntarily moving the eyes. The device, mounted on an eyeglass frame, includes a source of infrared light and a cadmium selenide detector fixed on the frame in such a way that, when the astronaut gazes straight ahead, the full power of the radiation infringes on the sclera on one side of the iris, and is reflected upon the detector; if, however, the astronaut deliberately turns the iris to the incident infrared beam, most of the power is absorbed and the controlling external relay closes, thereby setting out a predetermined sequence of operations.
The prior art also comprises a known device patented in 1969 (U.S. Pat. 3,473,868), consisting of an eyeglass frame which carries in front of each eye a modulated infrared radiation emitter and a pair of detectors, the circuit of the detectors measuring either the horizontal or the vertical displacements of an eye as a function of the variable reflecting power of the regions submitted to the incident radiation.
In practice however, this device is less satisfactory than simpler arrangements proposed by PFALTZ et al. Thus, the accurate setting of minute emitters and detectors is laborious and tedious and the whole operation has to be repeated every time the frame moves during testing and examination. Moreover, the proximity of objects close to the eyes is psychologically disturbing for the patient even if he is not dazzled; also, since the measurements have to be carried out in darkness to prevent troublesome interferences, the utility of the device is limited to experiments where the target of the eyes is only weakly lit, such as, the examination of reading power for example.
Finally, by virtue of the fact that the infrared light is modulated, the output signal only corresponds to an average of the positions of the regions submitted to illumination and not to the instantaneous state of said positions.
Further, as is well known, modulation invariably causes a delay of the corresponding signal, i.e. it is not fully on line, whereas a non-modulated system is. This is of course of considerable importance to certain types of measurements.